Your current CPU is good for 2 HD streams that need to be transcoded. Direct play on the other hand does not need remarkable CPU power. Plex itself says 2,000 Passmark for 10Mbit/s HD transcoding. Not one single HD stream is as low as 10Mbit/s here. The range is between 15 and 35 Mbit/s for untouched HD streams here.

If 4K ist on your list it becomes even heavier. Plex calls 12,000 to 17,000 Passmark for 40 - 50Mbit/s. Untouched 4K in reality is 45 - 65 Mbit/s with some even using higher values than that.

I bought used parts from eBay for example. An old X9Dri-F Supermicro board with 2x 2680 v2 CPUs. Give it a lot of used RAM and put transcoding directory on /tmp. These parts are cheap and good for 35,000 Passmark in total. Good for 2x 4K transcoding. If you stream local and Direct Play the old CPU would be enough.

Ah, and don't forget to check your Plex clients. For example a cheap Fire-TV 4K does Direct Play most of your content.